This invention relates to an apparatus for measuring the electrical conductivity of liquid, and particularly to an electrical conductivity measuring apparatus having a phase compensation circuit and suitable for the equipment based on a.c. voltage application.
Conventionally, a small change in the electrical conductivity of an electrolyte having a large electrical conductivity is measured by applying, to the background compensation resistor, a voltage equal in magnitude and opposite in polarity to the voltage applied to the measuring cell which contains the electrolyte, and by summing the output current from the resistor and the output current from the measuring cell.
This method is used mainly for measuring the electrical conductivity of a sample by detecting the variation of electrical conductivity of the solvent (electrolyte) before and after the sample is resolved in it.
In the above-mentioned conventional technique, the resistance of the resistor is adjusted before the occurrence of a small variation of electrical conductivity so as to nullify the output of the adder, so the varying component of the adder output is proportional to the varying component of electrical conductivity.
Techniques similar to the above-mentioned prior art are disclosed in JP-A-61-178457(U) and JP-A-62-167456, for example.
A method of measuring the electrical conductivity of solution based on d.c. voltage application is disclosed in JP-A-53-3872. This method is a 4-electrode method based on d.c. voltage application, and is said to be free from the variation attributable to the surface conditions of the electrodes and the influence of polarization phenomenon, allowing high accuracy measurement. The method is completely different in both principle and structure from the measuring method of the present invention.
A U.S. patent application Ser. No. 07/420,479 filed on Oct. 12, 1989 discloses an electrical conductivity measuring apparatus in which the above-mentioned resistor is adjusted automatically.